For reasons of safety and convenience, cellular radiotelephones often provide hands-free operation. By using a loudspeaker and external microphone the drivers of automobiles may engage in telephone conversations without having to take their hands from the steering wheel. A conventional hands-free system is illustrated in FIG. 1. FIG. 1 illustrates, a radiotelephone 130 connected to a remote loudspeaker 110 and a microphone 120 located within an automobile (represented by the box 100). Often, the volume of the loudspeaker 110 must be turned up quite loud for the driver to be able to hear the caller over ambient noise such as the noise of the engine, wind and road. Radiotelephones 130 still commonly use a moving coil loudspeaker 110. In fact, radiotelephones often have the loudspeaker placed in the handset of the radiotelephone, thereby restricting the size of the loudspeaker 110 to the space available in the handset.
FIG. 2 represents a cross-sectional schematic view of a moving coil loudspeaker. A flexible edge suspension 220 and a flexible center suspension 230 freely suspend a diaphragm element 270 in an open frame housing 290. The diaphragm 270 is nominally conical in shape. The diaphragm 270 is mechanically coupled to a voice coil 240 which is situated around a permanent magnet 280. Electrical audio signals (i.e., alternating currents of varying frequency and amplitude) are coupled to the coil 240 via a pair of wires 250.
The alternating currents of the electrical audio signals coupled to the coil 240 give rise to magnetic fields oriented either parallel or anti-parallel to the magnetic field of the permanent magnet 280. The orientation of the magnetic fields depends upon the direction of current flow through the coil 240. The magnetic fields created by the alternating currents in the coil 240 establish either forces of attraction or repulsion with respect to the magnetic field of the permanent magnet 280. Because of the mechanical coupling between the coil 240 and the diaphragm 270 and the freedom of movement of the diaphragm in the plane parallel to the forces created by parallel or anti-parallel magnetic fields (i.e. perpendicular to plane 215), changes in the direction and amplitude of the current generated by the electrical audio signal are translated into axial dislocations of the diaphragm 270.
Pressure waves generated by the axial dislocation of the diaphragm 270 propagate in the air as sound waves. For a given frequency, larger dislocations are associated with greater levels of volume. The magnitude of the sound pressure level, or volume, is directly related to the magnitude of the dislocation 210 of the diaphragm element 270 with regard to plane 215. To increase the volume of the sound (i.e., sound pressure levels) emanating from a dynamic loudspeaker such as that illustrated in FIG. 2, one simply increases the amplitude of the drive signal to the electrical contacts 250 which results in an increased deflection 210 of the diaphragm 270.
To faithfully reproduce sound, the loudspeaker 200 should have a relatively flat frequency response. In other words, over the loudspeaker's range of operating frequencies, electrical signals of the same amplitude should produce the same sound pressure level irrespective of the frequency of the signal. FIG. 3 illustrates a number of frequency responses for loudspeakers. The solid line 310 of FIG. 3 represents the ideal flat frequency response for a loudspeaker.
Unfortunately, physical realities give the loudspeaker 200 less than ideal frequency characteristics. Because of factors such as size and materials, most loudspeakers--especially low-cost versions--have sound reproduction characteristics that are a relatively strong function of frequency. The dotted line 320 of FIG. 3 shows an example of conventional loudspeaker characteristics. FIG. 3 also shows the resonant frequency 330 of the loudspeaker. As illustrated in FIG. 3, the resonant frequency 330 is the frequency at which the loudspeaker produces maximum sound pressure levels for a given input signal level.
As seen in FIG. 2, the diaphragm 270 is physically constrained with regard to the absolute magnitude 210 of dislocation. Overload or clipping of the loudspeaker occurs when the amplitude of the drive signal applied to the electrical terminals 250 would require the diaphragm 270 to move beyond the physical limitation of the suspensions 220, 230 or the open frame housing 290 of the loudspeaker. FIG. 4 illustrates that applying a fixed gain to a loudspeaker having the loudspeaker characteristic 410 shown by the dotted line results in the response shown at 420. From FIG. 4 it can be seen that loudspeaker overload will often first occur at the resonant frequency 330 of the loudspeaker. Loudspeaker overload occurs when the response exceeds the clipping level 400 as seen in FIG. 4. While illustrated as a linear function in FIG. 4 for purposes of illustration, the clipping level 400 is actually a highly non-linear and frequency dependent effect. Loudspeaker overload causes distortion of signals near the resonant frequency and produces harmonic overtones which interfere with audio signals at higher frequencies.
Use of a loudspeaker in a radiotelephone apparatus magnifies the phenomena of overload because the loudspeaker primarily reproduces human speech. As shown in FIG. 5, the power density spectrum for human speech has a definite bias for lower frequencies. Most of the energy of human speech lies at or near 500 Hz, however, the higher frequencies (i.e., 1000-3000 Hz), which are associated with the lower amplitude levels, provide most of the intelligibility. As can be seen from FIG. 5, the power density in human speech often closely matches the resonant frequency of the conventional loudspeaker used in a radiotelephone handset. For example, for loudspeakers of the type conventionally used within radiotelephone handsets, a resonant frequency of near 600 Hz is common. Therefore, volume levels which would not produce overload when reproducing music or other audio signals may result in overload when reproducing human speech.
Loudspeaker overload generates harmonic overtones in the higher frequencies where the intelligibility information of human speech exists. Overload causes wide-band distortion at loud volumes which combines with the informational higher frequencies of the audio signal and thereby makes it difficult to understand the speech. When overload occurs, increasing the amplitude of the audio signal does not increase the intelligibility of the speech carried in the audio signal. Thus, for example, the driver of an automobile faces a situation where turning up the volume to overcome the ambient noise of the automobile results in an unintelligible, albeit audible, garble.
The sound quality, or intelligibility of the voice signal can be improved somewhat by giving consideration to the design of the baffle and loudspeaker cavity. However, the limited space in a portable radiotelephone handset limits the availability of passive design choices such as the baffle and cavity design to eliminate the problems caused by overload of the loudspeaker.